The present invention relates to directly produced fused cast refractories which, as manufactured, are near in size and configuration to the desired final product as manufactured, and to a process for their manufacture which involves rapid melting and controlled, rapid cooling.
More particularly, the present invention is directed to a manufacturing process for near net shape fused cast refractories and to novel fused cast refractories which result from use of the present rapid melting, controlled rapid cooling process. The term "near net shape" as used herein means that the castings or moldings are near in size and configuration as cast or molded, and little or no material need be removed to prepare the moldings for use. The directly fusion cast refractories of the present invention are distinguished from those known in the art by a random microstructure throughout, as well as by other characteristics to be described herein. The term "random" as used herein in reference to microstructure means that the microstructure is non-directional in its crystal orientation.
Fused cast refractories have been known and used for many years. Such refractories have presented many advantages in certain uses over the older type of refractory products which comprise granular heat-resistant materials bonded in desired shapes with other heat-resistant ceramic materials. These older types of refractory moldings are also known as the "burned" and the "non-burned" types. These bonded refractories are produced by packing together refractory particles which have a large number of open pores between them. These particles are joined together by a bonding phase. When this refractory matrix is attacked by the action of an erosive or corrosive material, e.g., blast furnace slag, molten glass or the like, the fine particle portion of the matrix is predominantly eroded. Due to their interconnected porous nature, attack occurs beyond the exposed face of the refractory product.
A second classification of refractories is known as "fused cast" or "fusion cast" or "electrocast" refractories. These refractories have a very dense structure and hence high-strength and erosion resistance. They do not exhibit or contain interconnected pores.
A conventional fused cast refractory is manufactured by melting a mixture of the desired composition in an electric furnace similar to that used for the manufacture of fused alumina for use in abrasives. Such electric furnace includes a water-cooled iron or steel sheel without any lining other than that built up by the material being fused as it is fed into the furnace. Fusion is initially effected by heat from an electric arc between two or more electrodes inserted in the iron shell. After a bath of molten material is formed, the resistance of this material to passage of electric current through it is used to supply heat. The material to be melted is gradually introduced and the electrodes gradually raised as the fused mass accumulates. Apparatus of this type is shown and described in U.S. Pat. No. 929,517 to F. J. Tone. Following fusion of the ingredients, the molten material is cast into a suitable mold by tapping or tilting the furnace so that the molten mass flows into the mold. The molten material is heated to a temperature considerably above its melting point prior to casting. The mold is commonly made of graphite, although it may be formed of other suitable materials. The mold is provided with a riser or header of ample size to enable complete filling of the mold without interference by material freezing in the headers. The cast piece is left in its respective mold for heat treatment, or removed from the mold after the outer walls of the casting have solidified and then annealed without other than its own support. Such annealing generally takes several days and may be accomplished by covering the cast parts with hot sand or other insulating material. The cast parts are generally packed closer together, thereby allowing them to anneal by virtue of their own heat. Such annealing is necessary to prevent excessive localized shrinkage upon cooling which gives rise to stresses sufficient to result in fracture. Annealing may also be accomplished by placing the piece in a furnace and gradually reducing the temperature. After the pieces have cooled, the cast parts are inspected and finished by diamond cutting and/or grinding. The header may be removed shortly after casting or after annealing. The amount of material in the header is often about equal to the amount of material in the desired finished refractory piece. Header material is commonly recycled, but at considerable expense. The casting and annealing process is described in U.S. Pat. No. 2,279,260.
In known processes for the manufacture of fused cast refractories, the molten material is well above its fusion temperature when cast and a long cooling period is involved before complete solidification occurs. As a result, conventional fused cast products have a directional crystalline structure which is fine grained adjacent the surfaces of the casting and increasingly coarse grained directionally toward the center or last portion to cool. The crystalline structure and chemical composition of the casting changes from surface to center according to the rate of cooling and the liquid-solid phase diagram for the composition that was cast, all of which determine the nature of and the rate of advance of the solidification front. Additionally, the chemical composition of conventional fused cast refractories is not uniform due to stratification in the furnace and mold caused by heavier material such as ZrO.sub.2 and Cr.sub.2 O.sub.3 settling toward the bottom due to gravity. Typical cooling rates used in the manufacture of conventional fused cast refractories are of the order from about 10.degree. C. to about 50.degree. C. per hour. Also, carbon is introduced into the melt by the graphite electrodes of the electric arc furnace. This carbon can cause carbon monoxide and silicon monoxide evolution, resulting in pores in the conventional fused cast product. Also, it is known that conventional fused cast refractories contain carbon in amounts up to 1/2 percent. Carbon is undesired when the refractory is used as lining of glass manufacturing tanks.
Many different compositions for fused cast refractories have been previously suggested and it has been found that for specific uses certain compositions are superior. Such fused cast refractories are, for example, suggested in U.S. Pat. Nos. 2,063,154; 2,279,260; 2,911,313; 3,188,219; 3,232,776; 3,759,728; 3,773,531; 4,158,569; 4,490,474, as well as others. Any of these compositions, as well as many others, may be used in conjunction with the present invention.
The use of plasma to heat or treat materials is widely known for certain applications, as exemplified by the following.
U.S. Pat. No. 3,257,196 discloses an apparatus and process for treating a powdery refractory material by directly contacting it with a stream of plasma.
U.S. Pat. Nos. 3,429,962 and 3,645,894 describe the preparation of metallic oxide layers and bodies consisting thereof by plasma spray deposition of substantially spherically shaped agglomerate particles of metal oxides onto a metallic mandrel which is thereafter etched away. The process of the present invention is distinguished because a molten layer is maintained which serves to assist capture of the particles being thrown. In conventional plasma spray coating as described in U.S. Pat. Nos. 3,429,962 and 3,645,894, a high percentage, e.g., 30 to 40 percent of the particles thrown, are not captured because the surface is totally solid and must be to avoid melting of the metal substrate. Also, according to the prior art plasma spray techniques, the resulting ceramic layer is of high porosity.
U.S. Pat. No. 3,777,044 discloses a plasma arc furnace for remelting of sheet waste material of high reactivity metals and their alloys. The waste material to be melted is presented in the form of a consumable electrode.
U.S. Pat. No. 4,119,472 is directed to fired refractory articles formed of rebonded, fusion cast alumina/zirconia/silica refractory grain.
U.S. Pat. No. 4,426,709 describes a transferred arc plasma heated furnace for the production of steel from solid and/or liquid charging materials.
Koshi Kato, in an article entitled "Plasma Melting", published in Taikabutsu Overseas, Vol. 4, No. 2, 1984, generally describes plasma melting and many of its various applications, and several types of plasma melting furnaces. Among these is described a plasma progressive casting furnace which utilizes one or more torches to melt raw materials in a water-cooled crucible, the bottom of which is lowered as the melting process proceed to cool the molten metal successively from the bottom. In his conclusion, this author indicates that higher melting point materials such as ceramic will be processed by these furnaces in the future.